This type of sensor is known from DE-A-195 34 918. In the sensor described here, several sensor contacts are arranged next to a heating contact on a solid electrolyte tube. The arrangement disclosed here is well-suited for heating output in the middle or lower range, with the heater producing a gradual heating effect. If the sensor tube is heated unevenly, the thermomechanical tension may result in the destruction of the tube or may cause the contact surfaces to become detached from the sensor tube. The same type of sensor is known from DE-C-30 35 608; in this case, however, a wire-shaped heater is coiled on the sensor surface. A heater contact is fed through the inside of the sensor tube. In order to ensure a uniform grip on the sensor surface and to contact or feed the wire along the inside of the sensor tube, this type of arrangement must be relatively complicated, as such features may result in sealing problems at the point at which the wire is fed through the sensor material.
The use of tampon pressure to apply sensor contacts (external electrodes) to a cylindrical sensor object is known from DE-A-32 25 483. However, because of the relatively soft color transfer elements, tampon pressure is relatively inaccurate and completely unsuited for use in such devices as heater structures. Heater structures, such as those known from DE 195 34 918, produce uncontrolled temperature behavior at even minor dimensional variances because variances from the predetermined surface lead directly to variances in resistance.
In a sensor arrangement known from DE-C-36 28 572, the cap of the sensor tube is made of solid electrolyte material and a heater is arranged on an insulating material on the surface of the casing. These varying sensor surface structures produce the thermomechanical tension described in the published application, thereby creating the risk of destruction of the sensor.
A completely different sensor design is known from DE-C-44 01 793 or from DE-A-196 46 013. In this sensor design, a heating rod is arranged inside the sensor tube. The surface of the sensor tube is heated by thermal conduction beginning at the heating rod and passing through the atmosphere inside the sensor tube and through the sensor tube itself until the heat reaches the sensor external surface. Thus, this type of heating system is relatively sluggish. Because of the high thermal resistance between the sensor surface and the heater, it is impossible to precisely regulate the temperature of the sensor surface where the active sensor contacts (sensor electrodes) are located. Furthermore, if the position of the heating rod is not rotationally symmetrical inside the sensor tube, this sensor tube may be heated unevenly. The advantage of this arrangement, however, is that inaccuracies that occur when the heating conductor is pressed against the heating rod according to the procedure described in DE-A-32 25 483 are relatively unproblematic, as the heating rod itself is solid and thus relatively stable. Consequently, any inhomogeneous heating of the surface of the heating rod caused by the required thermal conduction to the surface of the sensor tube does not lead to an analogous inhomogeneity in the heating of the surface of the sensor tube.
The use of various processes to coat cylindrical substrates is also known in the art. For example, the use of screen printing to apply resistor layers of cylindrical layered resistors is known from EP-A-501 593 or from U.S. Pat. No. 4,075,968, where the resistance value itself is determined by the Greek key-shaped layers. The inhomogeneous distribution of the resistor layers on the substrates is not critical in these resistors, as such resistors exhibit low temperatures of about 100.degree., so that thermomechanical tension inside the substrate material is virtually nonexistent.
A lambda sensor is known from DE-A1-197 03 636 which comprises a sensor tube with a sensor contact arranged along its exterior and a layer of insulation positioned above the sensor contact; a heating contact is positioned on the insulation layer above the sensor contact. This arrangement is designed to achieve rapid heating of the sensor, thus ensuring that the sensor can become operational without a delay and can measure exhaust gas concentrations in motor vehicles when the exhaust gas is still cold. However, the disadvantage of this arrangement is that the gas to be measured comes into contact with the metallic heater before it reaches the sensor contact. When this occurs, the gas to be measured reacts with the metallic heater and does not reach the sensor contact in an unadulterated state. Consequently, the result of the measurement does not reflect actual conditions in the gas.
Based on the known state of the art, the objective of the invention is to provide a gas sensor that exhibits a short response time and that continues to operate precisely and with long-term stability in hot exhaust gases. In addition, the objective of the invention is to describe a process for the manufacture of this type of sensor.